We rely on our memories of past experiences, in conjunction with our current perception of the external world, to guide our behavior. The hippocampus is essential for rapidly encoding new memories. Subsequent processes between the hippocampus and distributed neocortical circuits are thought to consolidate these memories for long term storage and to retrieve stored memories to guide ongoing behavior. These varied mnemonic functions are thought to be subserved by distinct patterns of activity in the hippocampal subfields. In particular, the highly recurrent hippocampal area CA3 is crucial for the formation and consolidation of stored associations. As the associations formed in the recurrent CA3 network can only influence the neocortex via hippocampal output area CA1, understanding when how and when CA3 communicates with CA1 is critical for understanding memory processes. In Chapter 1 we investigate how CA3 and CA1 are coordinated during sharp wave-ripple events (SWRs) that are thought to support memory consolidation. We find that during SWRs there is a transient increase in the gamma synchrony between CA3 and CA1 and that the degree of synchronization between CA3 and CA1 is predictive of the quality of the replay of past experiences. Our results suggest that transient CA3-CA1 gamma synchronization is a central component of SWRs and this synchronization serves to clock the reactivation of stored memories across the hippocampal network. In Chapter 2 we investigate how the influence of CA3 on CA1 varies as a function of ongoing behavior. We find that as animals move faster, the influence of CA3 on CA1 decreases and that the level of coordinated spiking activity in CA1 reflects the influence of CA3. Our results suggest that movement speed drives a dynamic balance between learned associations and more independent sensory representations in the hippocampus which appears well suited to support the multiple

mnemonic functions of the hippocampal circuit.